A polyethylene-based resin foam is generally used as a packing material owing to its high elasticity and excellent oil and impact resistance. The polyethylene-based resin foam, however, has weakness that its stiffness and compressive strength are low. On the other hand, a styrene-based resin foam is excellent in stiffness, but has a weakness that it is brittle.
To overcome such weakness, Japanese Examined Patent Publication No. SHO 51(1976)-46138 (Patent Document 1), Japanese Examined Patent Publication No. SHO 52(1977)-10150 (Patent Document 2), Japanese Examined Patent Publication No. SHO 58(1983)-53003 (Patent Document 3) and Japanese Unexamined Patent Publication No. SHO 62(1987)-59642 (Patent Document 4) disclose methods in which polyethylene-based resin is impregnated with a styrene-based monomer and polymerization is performed to obtain styrene-modified polyethylene-based expandable resin beads.    [Patent Document 1] Japanese Examined Patent Publication No. SHO 51(1976)-46138    [Patent Document 2] Japanese Examined Patent Publication No. SHO 52(1977)-10150    [Patent Document 3] Japanese Examined Patent Publication No. SHO 58(1983)-53003    [Patent Document 4] Japanese Unexamined Patent Publication No. SHO 62(1987)-59642